The effective delivery of thrust to a projectile in a gun, or a projectile in the form of a rocket or the like, depends upon control of the ignition of the propellant. It is desirable to cause the energy of the burning propellant to be delivered within the time of interest, namely the time in which the projectile is subject to thrust from the propellant. Yet a complete and instantaneous detonation of all the propellant is destructive to the gun and does not maximize thrust. Preferably, the pressure acting on the projectile is substantially constant, thereby achieving maximum acceleration for a given bore pressure tolerance.
According to the state of the art, ignition and burning of propellant in conventional cartridges is controlled by the geometry of propellant grains. The shape, size and degree of perforation of solid propellant grains controls the rate of combustion once the propellant has been ignited by a fuse. However, these factors limit the energy density which can be packed into a cartridge and subsequently delivered to the projectile. For example, the conventional propellant RDX used in the art has a density of about 1.8 grams per cubic centimeter. It is typically pelletized into cylindrical pellets having a diameter of 3/8 inch, and a length of 1/2 inch, and is perforated. As a result, in the pelletized form necessary for controlled burning on a millisecond time scale, RDX has a density of about 1 gram per cubic centimeter. Furthermore, desensitizing agents are typically added to the propellant to further slow or control the combustion, which reduce the density to about half of the original density of RDX.
A variation on the conventional cartridge is the bulk liquid propellant cartridge, where a less sensitive, but also less potent liquid propellant is loaded at full density. Here, combustion rate is controlled not by grain size, but by the growth of a "Taylor Bubble" representing the interface between gaseous burn products and the unburned liquid. Unfortunately, the evolution of the bubble involves turbulent fluid dynamics as well as instability growth, and thus is not reproducible.
As an alternative to conventional cartridges, it is been attempted in the art to initiate and control the burning of propellant by means of electricity. Such cartridges have the potential to deliver far more impulse power than do conventional chemical cartridges because a higher energy density can be packed into the cartridge, and thrust can be delivered in a more timely and constant fashion to the projectile by means of the added control provided by electric current.
One method known in the art for burning propellant under the control of electric current requires striking an electric arc within one or more capillaries embedded in the propellant. Some measure of control is provided by the intensity of radiation impinging upon the ignited propellant, since the brightness may be controlled via the electric current. However, the degree of control is inversely dependent upon the ratio of chemically-generated to electrically-supplied energy. At one extreme is a conventional gun whose propellant has been ignited with an arc. This produces high efficiency, but with a burn rate determined entirely by the propellant. At the other extreme, all of the energy is provided electrically. This produces complete control over the pressure pulse, and allows one to choose an inert propellant of low molecular weight, allowing high velocities to be achieved. However, the efficiency with which the electrical energy is used to produce projectile kinetic energy is then very low.
Many electrically controlled designs suffer from some of the same problems as conventional liquid propellant cartridges, namely intrinsic irreproducibility in the dynamics of turbulent mixing and flame propagation over the distances involved in the cartridge. This means that while the supply of electrical energy is easily controlled, this control is negated in these designs by the random dynamics of propagating combustion fronts, plasma discharges or electrically-injected sprays.
For example, according to another method in the art, a propellant comprising two reactive components is ignited locally by using an electric arc to vaporize and then spray a fog of one atomized component into the other component locally. A number of such localized spray-type injections permits control of the propagation of the reaction throughout the cartridge. However, the electrical input requirements to obtain adequate mixing are considerable in this system, and it is thus not energy efficient. Furthermore, the system is unreliable and complicated because the spray dynamics are random and unreliable, and therefore achieve varying degrees of mixing between the two components.
In yet another method in the art, as described in U.S. Pat. No. 4,974,487 to Goldstein et al., a projectile is accelerated along a bore by plural plasma jet sources, located at different longitudinal positions along the length of the bore, and in the cartridge at the rear of the bore. The plasma jet is initiated in a low molecular weight dielectric material located in a discharge capillary with electrodes at each end. The plasma builds up a pressure through ohmic dissipation of its energy and passes through a fluid which may also be vaporized to contribute to the pressure front which propels the projectile. Disadvantageously, the device is subject to problems with the random and irreproducible dynamics of the plasma and its mixing with the fluid. While the current delivered to the capillary can be controlled, the behavior of the plasma in releasing the pressure build-up, the mixing of the plasma with the fluid, and the resulting vaporization of a component of the fluid are highly chaotic and problematic. Furthermore, in common with the other alternatives described above, a large amount of electrical energy is required to achieve the necessary plasma flow rates.
In a related device, described in U.S. Pat. No. 5,072,647 to Goldstein et al., a projectile is accelerated in response to high pressure gas such as hydrogen, generated in an exothermic reaction of a slurry of water and metal particles, initiated by a plasma discharge. The pressure of the hydrogen gas is maintained as the projectile accelerates down the gun bore by increasing the electric power applied to the plasma discharge. However, this design also suffers from the plasma dynamics problems associated with the aforementioned U.S. Pat. No. 4,974,487.
In U.S. Pat. No. 5,052,272 to Lee, an electric pulse is applied to a metallic wire to explode the wire into a slurry of aluminum particles in water, thereby igniting the slurry. Electrical energy continues to flow through the slurry and thereby augment the reaction. By these means the aluminum-water mixture is substantially reacted in the time of interest. However, no provision is made to control the rate of the reaction using electric current once the discharge of the ignition current is started. The exothermic reaction of the aluminum and hydrogen is promoted by the discharging electric pulse, without consideration of the position and rate of the reaction front. Furthermore, all of the propellant in the cartridge is reacted at once, leading to the same problems with the dynamics of flame propagation which plague the other aforementioned devices.
While the general concept of electrothermal chemical cartridges promises great improvement over conventional cartridges in the efficient and timely delivery of thrust to a projectile in a gun or rocket or the like, there is a need for a reliable means of using electric current to control the ignition of propellant. In particular, a cartridge is needed that avoids the problems associated with turbulent dynamics, has a reasonable electrical energy delivery efficiency, and performs reliably. It is furthermore desirable that such a cartridge be comparatively simple and cost-effective to construct.
The present invention advantageously addresses the above and other needs.